The human T lymphocyte CD2 (Tll) molecule is expressed early in thymic ontogeny and on most human T cells. Several lines of evidence suggest that CD2 is a receptor involved in T cell adhesion, activation, and differentiation. A ligand for CD2 has been found to be lymphocyte-function associated antigen-3 (LFA-3). The proposed project focuses on the functional role of CD2 during T cell activation and its interaction with its ligand, LFA-3. The goal of this project is to understand the regulatory role of the CD2 molecule in T cell activation and to delineate the structural requirements of CD2 for both LFA-3 binding and for signal transduction. Mutated CD2 molecules which lack LFA-3 binding sites and activation-related CD2 epitopes will be generated, expressed in an antigen-specific T cell hybridoma, and analyzed. Deletion mutants of the CD2 cytoplasmic domain will be generated and analy to help to define the structural requirements for signaling through CD2. The wild type and mutated CD2 molecules will be used to analyze the mechanisms of signal transduction. Soluble LFA-3 will be produced in an efficient expression system, and used to charac- terize LFA3 binding to CD2. Synthetic peptides based on the amino acid sequence of CD2 and, later, LFA-3 will be analy to define more precisely the binding site. This approach may ultimately lead to the design of small molecule agonists or inhibitors of the CD2/LFA- 3 interaction. There is accumulating evidence that the CD2/LFA-3 interaction can augment the antigen-specific response. LFA-3 ligand binding to CD2 may enhance or modulate T cell activation. The definition of the minimum requirements for the CD2/LFA-3 receptor-ligand interaction and for signaling may contribute to our understanding of the regulation and modulation of the immune response. Understanding this interaction may ultimately lead to novel immunotherapeutic approaches to enhance the T cell response to weak viral or tumor-specific antigens.